This patent specification is in the field of magnetic resonance imaging (MRI) using pulse sequences such as sequences making use of slice selective radiofrequency (RF) pulses.
Pulse sequences can be repeated with different RF pulse frequency to give resonance at different positions on a magnetic gradient to accomplish multi-slice coverage. Mansfield has described use of slice selective excitation. To increase signal recovery by T1 for higher SNR in images, Ernst has proposed that multiple slice planes can be obtained within the same repetition time (TR) of the sequence. In spin echo (SE) imaging, Crooks has proposed applying, within the same TR, frequency offsets to RF pulses (90xc2x0 and 180xc2x0) to accomplish multi-slice imaging, and this method is commonly used currently for many other types of pulse sequences. Some are known under the names gradient echo (GR), FLASH, EPI, SE-EPI, RARE, TSE, FSE, GRASE, BURST, and stimulated echo imaging.
The efficient interleaving of slices within the same TR of a SE sequence can, if time permits, be applied to interleaving of slices within the same TE, as proposed by Bishop in T2-weighted SE imaging. See Bishop, J. E. and Plewes, D. B. (Department of Medical Biophysics, University of Toronto), A new Multi-slice Technique Based on TE-Interleaving. The large time delays between RF pulses and SE signal can by utilized to record additional slices by interleaving multiple complete sequences of the 90xc2x0 and 180xc2x0 RF pulses, applied with different frequency offsets to independently excite and independently refocus signals from different slices within a shared TE time interval.
A very different method called BURST uses a set of low flip angle RF pulses applied on the same slice. This generates signals from fragments of the total magnetization. These fragmented or BURST signals are refocused with a constant unipolar gradient. The BURST technique creates an image extremely fast; however, the image is intrinsically lower in SNR than echo train sequences such as EPI or RARE. Unlike BURST, which refocuses fractions of the slice magnetization in each signal, the EPI and RARE sequences refocus the slice""s total magnetization into multiple signals.
An MRI process in accordance with the disclosure herein gives each of a number of excited slices a respective phase history different from those of the other slices. The process then simultaneously refocuses these slices, and acquires the MRI signals from them at respective different times related to the respective different phase histories of the respective slices. These MRI signals can be used as known to reconstruct respective images of the slices. This process can be called Simultaneous Image Refocusing (SIR).
For example, two consecutive 90xc2x0 RF excitation pulses at respective frequencies create magnetization in two slices s1 and s2 of a body, such as by using the known MRI practice of employing a main magnetic field and a slice selection gradient field acting on a body. A first dephasing pulse, on a readout axis Gr, is applied after the first but before the second 90xc2x0 RF pulse, thereby encoding the magnetization of the first slice (s1) but not of slice s2 since the second slice magnetization has not yet been created. A second dephasing pulse, on the same axis Gr, is applied after both slices have been magnetized by their respective 90xc2x0 FR pulses and, therefore, encodes the magnetization of both slices equally. This gives the two slices respective different phase histories relative to the Gr direction. If the subsequent readout pulses on the Or gradient axis are of unit area, and the first and second dephasing pulses on the Gr axis have areas of 0.50 and 0.25, respectively, the magnetization pathways they produce in the two slices will refocus echoes (MRI signals) within opposite halves of each read period, alternating with each EPI (echo planar imaging) gradient switching, due to the respective different phase histories of the two slices relative to the read gradient direction.
As one example, with equal signal bandwidth in conventional EPI (single slice) and SIR EPI (two slices), the echo trains are 30 ms and 45 ms, respectively, for 64 readout periods with linear phase encoding on a 1.5 T MRI scanner.
A significant advantage of the SIR pulse sequence disclosed herein is that it reduces the number of gradient switchings that cause physiologic neurostimulation and thereby impose performance limits on MRI data acquisition. When applied to EPI sequences, SIR with simultaneous refocusing of two slices improves the threshold to physiologic stimulation (pain) by a factor of approximately two. As is known, when conventional EPI is performed with very high performance gradients (high maximum (dB/dx) at greater net slew rate (dB/dx/dt) in the sequence) the neurostimulation of the body rather than gradient performance limits the data acquisition rate and number of slices imaged.
In pulse sequences such as RARE, and to some degree GRASE, typically 180xc2x0 RF pulses are used to refocus the signals, and RF dependent heating of the body ultimately limits the imaging speed and limits the number of slices acquired during an image sequence. In RARE there typically is a one-to-one relationship between the number of signals and 180xc2x0 RF pulses. An advantage of the SIR technique applied to RARE sequences, when for example two slices are acquired in a read period, is to reduce to half the number of 180xc2x0 degree RF pulses per slice. This greatly reduces RF heating of the body and therefore permits faster image acquisition using a shorter TR or a larger number of slices in the same time as RARE, or tradeoffs of shorter TR or more slices in the same time period.
While for simplicity most examples discussed herein involve two simultaneously refocused slices, it should be clear that when more than two slices are encoded simultaneously in SIR, there is a further reduction of RF heating per slice. It should also be clear that the techniques described herein as applied to slices can be applied to volumes having a greater thickness than that of a typical MRI slice, and that the multiple simultaneously refocused slices need not be parallel to each other, and need not be perpendicular to a longitudinal axis of the MRI magnet. Each slice in SIR can be at its own angle in space, defined for example by the known technique of using a respective combination of concurrent gradients along different axis.
After SIR EPI data acquisition of two slices, xe2x80x9ctime reversalxe2x80x9d of data on alternate polarity read gradients separates data from the two slices onto two halves of the frequency axis of k-space. This acquired k-space is separated into two k-spaces, by directly dividing it in half on the frequency axis. Another method of separating the k-spaces is by a deconvolution method which separates the high spatial frequency k-space data overlapping in time. In fact, there is an extension of the two highest resolution regions of k-space of two slices, or leakage of low amplitude, highest spatial frequency k-space data between the two slices k-space.
Phase alternation of one of the two 90xc2x0 RF excitation pulses can be used in two acquisitions of the same sequence to create a polarity change in one of the two simultaneously acquired slices"" k-space. Taking different linear combinations (i.e. adding or subtracting) the two phase alternating data sets nulls data from one or the other slice. The resulting highest spatial frequency data can therefore be utilized in a larger k-space data matrix with advantages of higher spatial resolution, higher SNR/unit volume, and more complete separation of signal leakage between two simultaneously acquired slices. Another similar approach to obtaining different combinations and dependent manipulation of signals is to change the order of slice excitation with the 90xc2x0 pulses in multiple average acquisitions (i.e., multiple acquisitions of the entire imaging sequence). This produces different linear combinations of k-space overlaps, allowing for separation of k-spaces of different slices.
Applied to RARE type sequences, SIR uses at least two different slice selective pulses, typically but not necessarily of 90xc2x0 each but at different frequencies, f1 and f2; however, a single refocusing pulse, typically of 180xc2x0, centered at (f1+f2)/2 is used with broader slice profile which overlaps onto the positions of the excitation slices and so the 180xc2x0 pulse refocuses both slices simultaneously. In the longer echo train, the signals of the two slices alternate in their refocused position in each read gradient. With appropriate timing between the RF pulses and appropriate Gr dephasing pulses, all the signals can be acquired with nulling of field inhomogeneity and susceptibility errors. With this approach in SIR, two or more spin echoes can be refocused between each pair of 180xc2x0 pulses, rather than a gradient echo and spin echo. Given the alternating order of the slice signal pairs, the T2 decay curve is sampled discontinuously rather than incrementally in time. Discontinuous sampling of the T2 decay curve in the GRASE sequence, has shown that discontinuous sampling does not create image artifacts nor does it significantly change the image contrast. The CPMG timing of spin echoes from the two slices is replaced with an alternating short and long 90xc2x0-180xc2x0 time interval; nevertheless, all signals are spin echoes. The initial three RF pulses therefore do not have CPMG timing; however, this timing has been tested and it does not create interference between spin echoes and stimulated echoes. The resulting image acquisition uses half the number of 180xc2x0 RF pulses, which reduces body heating and permits twice the number of slices or a net faster image acquisition.
Bandwidth of signals increases when they are sampled more quickly in the SIR sequence disclosed herein, without changing the duration of the read period from that in the otherwise similar conventional RARE or EPI sequence. With implementation of simultaneous image refocusing in EPI or RARE, there is increased net ADC sampling time per total sequence time. In the exemplary two slice SIR EPI, there are half as many read gradient switchings in acquiring two slices. In RARE there are half the number of Gp phase encode pulses, Gp rewind pulses and 180xc2x0 RF pulses in the case of simultaneously refocusing two slices. If signal bandwidth is maintained the same as in a conventional sequence, due to the above reduction of RF and gradient pulses the total data acquisition of two slices is reduced. For example, an EPI image is encoded in an echo train of 30 milliseconds and with SIR two EPI images are encoded in a 45 millisecond echo train. When EPI is used for functional MRI, BOLD contrast is created using a 30 millisecond delay between the excitation 90xc2x0 pulse and the echo train for a total of 60 milliseconds per slice. Using SIR, two slices are encoded in 75 milliseconds. With merely 25% longer acquisition time, SIR gives two BOLD weighted images instead of one. The average speed of multi-slice EPI imaging is increased substantially; however, this must be adjusted or optimized with the associated effects of increased echo spacing and increased T2* decay which increase distortions and susceptibility artifacts, especially in gradient echo EPI.
With the SIR technique, total imaging time is reduced even more when any xe2x80x9cpreparatory periodxe2x80x9d is used. The preparatory period is when gradient pulses, delay time or RF pulses are incorporated into the sequence to encode physiologic or physical information in the image. One commonly used pulse sequence preparation is the Stejskl-Tanner gradient pulse sequence used to obtain diffusion weighted (DW) EPI imaging. The DW preparatory period requires a relatively large proportion of sequence time, 80 to 100 milliseconds, on current clinical MR scanners. The total acquisition time of each single shot DW SE-EPI image is about 150 milliseconds. With the simultaneous imaging sequence (SIR) disclosed herein, the time of two DW EPI images would be approximately 180 milliseconds as they would simultaneously share a DW preparatory period.
In the case of a preparatory period encoding velocity of blood or motion in general, there is the advantage of obtaining a measure of velocity in multiple slices at the same time. This eliminates or greatly reduces inaccuracies due to changes in position and in velocity due to pulsating or respiratory motion in the body. Such simultaneous velocity images are obtained by using a bipolar gradient pulses, Stejskl-Tanner gradients, or other gradient pulses in the preparatory period.
SIR can also be applied to stimulated echo sequences. In a conventional stimulated echo (STE) sequence, a first 90xc2x0 RF pulse excites a slice, a second 90xc2x0 RF pulse stores the signal on the longitudinal axis, where the gradient pulses cannot encode the signal, and a third 90xc2x0 RF pulse returns the signal to the transverse axis, where gradients can encode the signal and the signal can be read with Gr gradients.
When SIR is applied to an STE sequence, in the case of two slices s1 and s2, a first RF pulse that typically is 90xc2x0 excites both slices. A first Gr dephasing gradient encodes the signals from both slices. A second, more selective 90xc2x0 pulse stores the signal from slice s1 but not from slice s2 on the longitudinal axis. A second Gr dephasing gradient is applied and encodes only the signal from slice s2 (because slice s1 has experienced two 90xc2x0 pulses and is stored on the longitudinal axes). A third 90xc2x0 pulse stores the signal from slice s2 on the longitudinal axis. A fourth 90xc2x0 pulse affecting both slices s1 and s2 returns to the transverse axis (or refocuses) the signals from both slices s1 and s2. The signals can be read out by EPI alternation of Gr polarity, creating an alternating order of signals from slices s1 and s2. Additional Gp slice encoding gradients can be applied during the echo train, in the conventional way, to encode each set of signals from s1 and s2 differently for 2D FT. This method of SIR STE can be generalized to more than two simultaneously refocused slices. Also, the order of Gr encoding can be changed, e.g., the first Gr dephasing pulse can be applied with opposite polarity immediately after the fourth 90xc2x0 pulse and have same or comparable effect.
The simultaneous images refocused (SIR) technique is believed to be most successful, at least in part, because of three preferred features of the exemplary image sequence (which may be performed in various subcombinations and permutations).
I. Echo Generation
First, as previously mentioned, a set of slice selective RF excitation pulses creates signals present together in the pulse sequence. Different combinations of dephasing gradient pulses on the Gr axis are applied between the excitation RF pulses. These Gr pulses give the signals different phase history. Different linear combinations of net Gr dephasing are possible because of two fundamental principles: 1) gradient effects are not encoded in the signal from a slice if the gradient pulse is applied before the slice""s selective RF excitation, and 2) a Gr pulse applied after the time of all of the slice selective RF excitations causes dephasing of signal equally in all slices. Specific linear combinations of dephasing Gr pulses allow the signals from different slices to be moved to different locations on a read gradient so that the signals are not superimposed in time.
In some situations it may also be desirable to create different k-space phase encoding ordering in different slices which can also be established by applying different phase gradient pulses (Gp) between the excitation RF pulses. In general terms, gradient pulses applied after all of the RF excitations of slices encode all slices equally and simultaneously, whereas gradient pulses applied between the RF excitation pulses encode only the earlier excited slices.
II. Echo Refocusing
The echoes from different slices are simultaneously refocused by switching the polarity of read gradients as in EPI or by RF refocusing pulses. The RF refocusing pulse can be simultaneously experienced on all image planes by making it broader in width than the excitation pulses onto which it overlaps. The Gp phase encode pulses encode the slices simultaneously. There is a significant reduction in the amount RF energy absorbed by the body as compared with RARE or GRASE sequences because a fractional number of 180xc2x0 RF pulses is needed to create the same number of image slices.
III. Preparation Period
The encoding of different physiologic or physical information in the image; i.e., diffusion, velocity or brain activation with blood oxygen level dependence (BOLD) contrast, occurs in a preparation period (PP) in the pulse sequence prior to signal readout. In SIR technique, the PP is applied simultaneously to multiple slices. This reduces the number of PP periods by a factor equal to the number of simultaneous images. In the case of BOLD and Diffusion PP there is a large time saving. In velocity, the physiologic measurements is made simultaneously in 3D space (two adjacent slice planes) rather than in 2D image. This gives temporal coherence of velocity information in the presence of cardiac and respiratory dependent motions of the human body""s organs, blood and cerebrospinal fluid.
The above description of SIR technique is general to both multi-slice 2D FT imaging and multi-volume 3D FT imaging, with the above three principles generally applicable to both closely related imaging methods. Furthermore, the images and their preparatory information are not required to be constrained as parallel in space as they can be oriented in any angle in three dimensional space by simultaneously applying different gradients creating different vector components of three dimensional space for slice selective excitation, readout or other encodings.
The description of the SIR technique is general to different flip angles of RF excitation pulses and RF refocusing pulses. Throughout the description of SIR technique, 90xc2x0 excitation pulses and 180xc2x0 refocusing pulses are used in an exemplary fashion.
In this general description of the SIR technique, constant amplitude read gradients (Gr) are used; however, variations in the Gr pulse can be performed, for example, to create differences in signals bandwidths.